Why do couples stay together?

Wondering about the answer to this question because one of my male friends has a really moody girlfiend. Don't know how he tolerates that. One minute they are fine and the next, she is picking a fight with him just because she is in a bad mood or has PMS.

Feel sorry for my friend. I hate it when a guy yells at me or tries to pick a fight just because he had a crappy day. I would rather walk away and wait until he calms down before sorting things out.

So why does my friend stay with her? He says it's because there are more good times than bad times. And she has some qualities that he really likes.

But still......I wonder if that is enough?